Simultaneous brazing and corrosion protecting refractory metals



United States Patent 3,132,928 SIMULTANEOUS BRAZING AND CORROSIONPROTECTING REFRACTORY METALS Donald D. Crooks, San Jose, and Robert E.Wallace, Los

Altos, Calif., assignors, by mesne assignments, to the United States ofAmerica as represented by the Secretary of the Navy No Drawing. FiledFeb. 26, 1962, Ser. No. 175,852

2 Claims. (Cl. 29-198) The present invention relates to the coating andbrazing of shaped objects of refractory metals so as to render theobjects resistant to corrosion at high temperatures.

In the recent past the various alloys of steel have sat isfied moststructural needs. However, with the advent of rockets and missiles thetemperatures involved that will have to be sustained by the metalstructural members and parts has increased several fold.

The strength of a metal is directly related to its melting point.Therefore, the refractory metals would be a preferred material ofconstruction in the rocket and missile field because of their highmelting points. Refractory metals considered most promising aremolybdenum (M.P. 2620 C.), tantalum (M.P.2996i50 C.) and tungsten (M.P.3370 Q).

As with most materials, one will find that in order to take advantage ofcertain properties, one will have to compensate for other properties ofthe material.

These materials are subject to drastic corrosion or oxidation whenexposed to oxidizing atmospheres at temperatures over 1400" F. It hasbeen suggested that components for rocket and missile applications couldbe fabricated from these refractory metals if the external surfacesexposed to oxidation could be provided with a protective coating toexclude the oxidizing atmosphere.

Many different coatings have been applied to these metal articles in anattempt to prevent this high temperature oxidation. One in wide use isthe silicide coating. Such a coating may be produced by vapor depositinga thin layer of silicon onto the hot surface of the transition metal orit can be formed by being painted or sprayed on, or by the sinteredmethod. While such a coating has very good protective value at elevatedtemperatures, its brittleness may result in cracking when struck ordeformed. In order to be of value, any protective coating mustcompletely cover the surface. Any defects, such as a crack or pinholemakes the coating useless since the base metal will erode by oxidationat the point of defect.

Other suggested methods of protection for these refractory metals arethe application of ceramic coatings and cladding. These latter methodsare for limited application and have not found any general use.

Accordingly, it is an object of the present invention to provide amethod of protectively coating refractory metals.

Another object is to provide a means of bonding together shaped objectsof refractory metals.

Still another object of the invention is to provide a method ofsimultaneously protectively coating and bonding together shaped articlesof refractory metals.

Yet another object is the provision of a novel corrosion resistant,brazing alloy for coating surfaces.

Other objects and many of the attendant advantages will be readilyappreciated as the same becomes better understood by reference to thefollowing description of the invention.

The refractory metal to be coated and/or brazed is first prepared forelectroplating by suitable known methods. It is then plated with nickelfollowed by an electroplate of rhodium.

The plated metal object is then heated in a protective vacuum atmosphereto about 2500 F. The protective atmosphere can also be hydrogen orcracked ammonia "ice gas. The binary coating melts and fuses to the baserefractory metal. The result is a high temperature corrosion andoxidation resistant integrally bonded coating. The integral bond is aresult of the nickel simultaneously forming solid solution alloys withthe refractory metal and rhodium electroplate. The solid solution formsmos-t readily when tantalium is the base metal being coated.

If desired, these coated refractory articles can be joined by placingthem in close proximity or wiring them together and then heating toabout 2500 F. in vacuum or other protective atmosphere. The adjoiningelectroplates will then fuse together resulting in the refractory metalshapes being brazed together. This process will coat and protectintricate and complex shapes as readily as simple configurations. Thecoated and brazed articles of this invention have been tested to 2800 F.in a dynamic oxidizing atmosphere and have shown excellent resistance tooxidation or corrosion.

The following is a more detailed description of an embodiment of thepresent invention. The refractory metal article to be protected in thisexample tantalum, is first degreased by being immersed in a suitablesolvent such as acetone or carbon tetrachloride.

The tantalum article is then cleaned mechanically by abrasive means suchas emery paper, steel wool, etc.

Then, the article is cleaned chemically by etching in an appropriateacid such as a solution of sulfuric acid nitric acid (4.5%),hydrofluoric acid (5%) containing 18.8 grams per liter of chromic oxide.

The tantalum article is then placed directly in a conventionalacid-nickel strike bath for several minutes at a cathode current densityof milliamperes per square centimeter. The specimen is then transferreddirectly without rinsing to a conventional Watts-type nickel platingsolution where plating is continued at .5 to2 amperes per squaredecimeter to a thickness of about .0001 inch to .001 inch depending ondesired final results.

The nickel coated tantalum specimen is then removed from this bath,rinsed and placed in a rhodium plating bath where rhodium is depositedat a current density of 7 amperes per square decimeter to a thickness of.0001 inch or less, again depending on desired final results. The coatedarticle is removed and heat treated and/ or brazed as previouslydescribed.

The several solutions used are conventional and the composition thereofmay be varied within the skill of the plating art. The solutions used inthe example herein for the nickel bath consisted of an aqueous solutioncontaining about 240 grams per liter of NiSO -6H O, about 45 grams perliter of NiCl -6H O and 30 grams per liter of H BO the remainder beingwater. The rhodium bath consisted of 162 grams per liter of thephosphate or sulfate of rhodium with an excess of acid of eithersulfuric or phosphoric acids.

It should be understood that the foregoing disclosure relates only to apreferred embodiment of the invention and that numerous modificationsmay be made therein without departing from the spirit and the scope ofthe invention as set forth in the appended claims.

What is claimed is:

1. A composite article including a base of a refractory metal selectedfrom the group consisting of tungsten, tantalum and molybdenum having amultiple layer metal surface coating fused to said base to providehigh-temperature oxidation and corrosion resistance, said surfacecomprising rhodium integrally bonded to said base metal by solidsolution alloys of base metal, nickel and rhodium, said integrallybonded surface being characterized by alloyed strata resulting from aplating of said base with thin coatings of nickel and rhodium andsubjection of said article to heat treatment at about 2500 F whereby afused integrally bonded surface is obtained on said base.

2. The method of protecting refractory metal selected from the groupconsisting of tungsten, tantalum and molybdenum articles againstoxidation and corrosion at high temperatures including the steps ofcleaning said metal articles, electroplating nickel thereon of athickness of from about .0001 to .001 inch, thereafter electroplatingrhodium up to about .0001 inch in thickness on the nickel surface, andheat-treating said plated article in a protective non-reactiveenvironment at about 2500 P. whereby solid solution alloys of nickelwith the refractory metal and rhodium are formed, thereby integrallybonding an oxidation and corrosion resistant coating on said refractorymetal article.

References Cited in the file of this patent UNITED STATES PATENTS2,698,913 Espersen Ian. 4, 1955 2,719,797 Rosenblatt et a1. Oct. 4, 19552,798,843 Slornin et a1. July 9, 1957 FOREIGN PATENTS 814,644 GreatBritain June 10, 1959 OTHER REFERENCES Metal Industry, March 26, 1948,pp. 249, 250 and 254.

1. A COMPOSITE ARTICLE INCLUDING A BASE OF A REFRACTORY METAL SELECTEDFROM THE GROUP CONSISTING OF TUNGSTEN, TANTALUM AND MOLYBDENUM HAVING AMULTIPLE LAYER METAL SURFACE COATING FUSED TO SAID BASE TO PROVIDEHIGH-TEMPERATURE OXIDATION AND CORROSION RESISTANCE, SAID SURFACECOMPRISING RHODIUM INTEGRALLY BONDED TO SAID BASE METAL BY SOLIDSOLUTION ALLOYS OF BASE METAL, NICKEL AND RHODIUM, SAID INTEGRALLYBONDED SURFACE BEING CHARACTERIZED BY ALLOYED STRATA RESULTING FROM APLATING OF SAID BASE WITH THIN COATINGS OF KNICKEL AND RHODIUM ANDSUBJECTION OF SAID ARTICLE TO HEAT TREATMENT AT ABOUT 2500*F. WHEREBY AFUSED INTEGRALLY BONDED SURFACE IS OBTAINED ON SAID BASE.
 2. THE METHODOF PROTECTING REFRACTORY METAL SELECTED FROM THE GROUP CONSISTING OFTUNGSTEN, TANTALUM AND MOLYBDENUM ARTICLES AGAINST OXIDATION ANDCORROSION AT HIGH TEMPERATURES INCLUDING THE STEPS OF CLEANING SAIDMETAL ARTICLES, ELECTROPLATING NICKEL THEREON OF A THICKNESS OF FROMABOUT .0001 TO .001 INCH, THEREAFTER ELECTROPLATING RHODIUM UP TO ABOUT.0001 INCH IN THICKNESS ON THE NICKEL SURFACE, AND HEAT-TREATING SAIDPLATED ARTICLE IN A PROTECTIVE NON-REACTIVE ENVIRONMENT AT ABOUT 2500*F.WHEREBY SOLID SOLUTION ALLOYS OF NICKEL WITH THE REFRACTORY METAL ANDRHODIUM ARE FORMED, THEREBY INTEGRALLY BONDING AN OXIDATION ANDCORROSION RESISTANT COATING ON SAID REFRACTORY METAL ARTICLE.